For me, it's easily the Royal Knights arc. When I first went on forums that talked about Digimon, I was actually surprised hearing about how so many people hated the ending to 02. I still remeraber feeling fairly happy about it. Sure, in retrospect, it was corny and a good chuck of the kiRAB' future didn't make a lick of sense, but I thought that it was kind of cute. It was nice to see that even into their adult years, all of the chosen children were still frienRAB and got together with their own kiRAB. I should rewatch that episode on Crunchyroll at one point to see if I still feel like that though.
With the Royal Knights arc, it just felt boring to me. They kept on losing over and over again and unlike with the BlackWarGreymon arc, the villains weren't interesting. Even the plots for most of the episodes were fairly dull. It did lead to a pretty decent finale though, which I think is the only part of the arc that I appreciated.